1. Technical Field
The present invention relates to a hydrogen separation device that extracts hydrogen from a hydrogen-containing gas, as well as to a fuel cell system.
2. Description of the Related Art
Fuel cells that generate electric power through electrochemical reactions of hydrogen and oxygen have recently gathered attention as the novel energy source. Hydrogen supplied to the fuel cells is obtained, for example, by reforming a hydrocarbon material like methanol to a reformed gas and extracting hydrogen from the reformed gas by means of a hydrogen separation device.
A known structure of the hydrogen separation device uses a hydrogen separation member that is made of, for example, palladium or a palladium alloy and allows selective transmission of hydrogen. In this hydrogen separation device, a supply of the reformed gas is fed to one face of the hydrogen separation member, while hydrogen is extracted from the other face of the hydrogen separation member.
The hydrogen extracted by the hydrogen separation device is carried on a hydrogen carrier gas called a purge gas and is supplied to hydrogen electrodes (anodes) of the fuel cells. The purge gas may be, for example, steam, the remaining reformed gas after separation of hydrogen (non-transmitted gas), or an anode off gas discharged from the hydrogen electrodes of the fuel cells. A cathode off gas discharged from oxygen electrodes (cathodes) of the fuel cells is also applicable to the purge gas, as disclosed in Japanese Patent Laid-Open Gazette No. 2001-283885.
Other techniques relating to separation of hydrogen from the reformed gas are also disclosed in Japanese Patent Laid-Open Gazette No. 2002-83617 and No. 2001-223017. The former technique releases steam from the anode off gas of fuel cells and utilizes the released steam as the purge gas. The latter technique supplies steam to a hydrogen separation unit to lower the hydrogen partial pressure and thereby enhance the hydrogen separation efficiency.
In the case of using the cathode off gas as the purge gas, it should be noted that the cathode off gas may contain unreacted oxygen, which has not been consumed in the fuel cells. When oxygen remains in the purge gas, a reaction of hydrogen with oxygen may proceed in the hydrogen separation device. Its reaction heat may interfere with the sufficient hydrogen permeation of the hydrogen separation member. The reaction may also deteriorate the hydrogen separation member.
The object of the invention is thus to prevent reduction of hydrogen permeability and deterioration of a hydrogen separation member in a hydrogen separation device that uses a cathode off gas as a purge gas.